Inverter with AC forward bridge and improved DC/DC topology

1. A DC-to-AC power converter having a main DC input and a main single-phase AC output, configured to convert and adapt a DC voltage at the main DC input into a sinusoidal AC voltage of a fundamental frequency at the main AC output and to deliver a rated power at the main AC output to a load, comprising: a single DC-to-DC converter having as input the main DC input and having a DC output and a tank capacitor being connected to the DC output, two low frequency diodes biased so as to be configured to pass current from, respectively to, the DC output to, respectively from, the tank capacitor; according to a direct path, a bidirectional voltage-type DC-to-AC converter in cascade with the DC-to-DC converter, the bidirectional voltage-type DC-to-AC converter having a DC input-output connected to the DC output and an AC output-input connected to the main AC output; according to a bypass path, and in parallel to the bidirectional voltage-type DC-to-AC converter and to the low frequency diodes, a current-type low frequency full switching H-bridge comprising an AC forward bridge, having a DC input and an AC output, the DC input being connected to the DC output of the single DC-to-DC converter and the AC output being connected in parallel to the main AC output, the AC forward bridge having a working frequency less than 1 kHz; and a controller configured to control the bidirectional voltage-type DC-to-AC converter to deliver at the first AC output-input the sinusoidal AC voltage and to control the AC forward bridge to deliver a quasi square-type AC forward current in phase with the sinusoidal AC voltage, the control device being configured to control the bidirectional voltage-type DC-to-AC converter and the AC forward bridge, so that the latter are operable simultaneously, wherein, when an instantaneous voltage between the terminals of the main AC output attains a predetermined level, the low frequency forward AC bridge is switched on, the low frequency diodes being reverse biased and non-conducting and a constant power is supplied by the DC-to-DC converter directly to the load.

2. The DC-to-AC power converter according to claim 1 , wherein the two closed switches of the low frequency AC forward bridge are selected depending of a polarity of the output AC voltage.

3. The DC-to-AC power converter according to claim 1 , wherein the DC-to-DC converter is designed configured to support a variable output voltage while transferring a nearly constant power.

4. The DC-to-AC power converter according to claim 1 , wherein the DC-to-DC converter is isolated and comprises, at a primary side of the transformer, an active clamp, comprising a main MOSFET connecting the primary winding of a transformer to a primary source providing the main DC input, a resonance capacitance being in parallel with the primary MOSFET to allow the primary MOSFET to operate in ZVT, and a capacitance and a second MOSFET configured to provide voltage clamping on the main MOSFET and consequently protect the latter against overvoltage.

5. The DC-to-AC power converter according to claim 4 , wherein the DC-to-DC converter further comprises, at a secondary side of the transformer, at least a first capacitor creating an AC connection to a secondary winding of the transformer, a resonance inductance that is reducible to a leakage inductance of the transformer, a rectifying diode and a free-wheeling diode configured to rectify a voltage created at the secondary of the transformer and a secondary resonance capacitor connected in parallel with the rectifying diode and a decoupling capacitor in parallel with terminals of the output DC voltage.

6. The DC-to-AC power converter according to claim 5 , wherein the first capacitor, the resonance inductance, the rectifying diode, and the free-wheeling diode are configured so that, during a magnetization phase of the transformer, the input voltage of the converter, VIN, reflected to the secondary of the transformer as NVIN, with N being a transformer turn ratio, charges the first capacitor and creates a resonance between the latter and the resonance inductance, through the rectifying diode, the free-wheeling diode being non conducting, and without overvoltage at a junction between the rectifying diode and the free-wheeling diode.

7. The DC-to-AC power converter according to claim 5 , wherein the first capacitor, the resonance inductance, the rectifying diode, and the free-wheeling diode are configured so that, during a demagnetization phase of the transformer, a current flows from the charged first capacitor to the load through the resonance inductance and the free-wheeling diode, the current transferring not only a magnetization energy of the transformer but also, simultaneously, an energy stored during the magnetization phase in the first capacitor.

8. The DC-to-AC power converter according to claim 7 , wherein an output power (P O ) of the DC/DC converter is related to an output power (P FB ) of an equivalent flyback converter by the following equation: P O = P FB ⁢ M , where ⁢ ⁢ M = V O V O - NV I ⁢ N , wherein V IN and V O are respectively input and output voltages of the DC/DC converter, M being a multiplication factor higher than 1.

9. The DC-to-AC power converter according to claim 8 , wherein M is higher than 2.

10. The DC-to-AC power converter according to claim 1 , wherein the DC-to-AC power converter is bidirectional.

11. The DC-to-AC power converter according to claim 1 , wherein the DC-to-DC converter is isolated and comprises, at a primary side of the transformer, a two-transistor forward converter primary stage, comprising two MOSFETs, each directly connecting at one respective end thereof an end of the primary winding of a transformer and a respective primary source terminal providing the main DC input, a resonance capacitance being in parallel with each MOSFET respectively, to allow the MOSFETs 4 W operate in ZVT, and diodes arranged to respectively connect each of the main MOSFETs to the primary source terminal which is not the primary source terminal directly connected to the corresponding MOSFET respectively.

12. The DC-to-AC power converter according to claim 1 , wherein the AC forward bridge has of a working frequency of 400 Hz or 50/60 Hz.

13. The DC-to-AC power converter according to claim 1 , wherein the DC-to-DC converter is non isolated.

14. A DC-to-AC power converter having a main DC input and a main single-phase AC output, configured to convert and adapt a DC voltage at the main DC input into a sinusoidal AC voltage of a fundamental frequency at the main AC output and to deliver a rated power at the main AC output to a load, comprising: a single DC-to-DC converter having as input the main DC input and having a DC output and a tank capacitor being connected to the DC output, two controlled switches configured to pass current from, respectively to, the DC output to, respectively from, the tank capacitor; according to a direct path, a bidirectional voltage-type DC-to-AC converter in cascade with the DC-to-DC converter, the bidirectional voltage-type DC-to-AC converter having a DC input-output connected to the DC output and an AC output-input connected to the main AC output; according to a bypass path, and in parallel to the bidirectional voltage-type DC-to-AC converter and to the low frequency diodes, a current-type low frequency full switching H-bridge comprising an AC forward bridge, having a DC input and an AC output, the DC input being connected to the DC output of the single DC-to-DC converter and the AC output being connected in parallel to the main AC output, the AC forward bridge having a working frequency less than 1 kHz; and a controller configured to control the bidirectional voltage-type DC-to-AC converter to deliver at the first AC output-input the sinusoidal AC voltage and to control the AC forward bridge to deliver a quasi square-type AC forward current in phase with the sinusoidal AC voltage, the control device being configured to control the bidirectional voltage-type DC-to-AC converter and the AC forward bridge, so that the latter are operable simultaneously, wherein, when an instantaneous voltage between the terminals of the main AC output attains a predetermined level, the low frequency forward AC bridge is switched on, the low frequency diodes being reverse biased and non-conducting and a constant power is supplied by the DC-to-DC converter directly to the load.

15. The DC-to-AC power converter according to claim 14 , wherein the controlled switches comprise MOSFETs, IGBTs, or relays.